In the North American emergency services Network, the location of a caller is conveyed to a Public Safety Answering Point (PSAP), a physical location where emergency telephone calls are received, in the form of a telephone number of the calling party. The PSAP uses an Automatic Location Identification (ALI) database, which is pre-populated with the specific location of the caller in order to direct emergency responders (police, fire, ambulance, and rescue) to reach the caller. Voice over IP (VoIP) telephony, however, uses the Internet to make telephone calls, thereby complicating the process of locating a caller due to the mobility of the VoIP telephony devices and corresponding transmission control protocol/Internet protocol (TCP/IP) addresses, i.e., the location of the caller is not static. With the increasing use of VoIP telephony for communications, various governmental bodies, such as the Federal Communication Commission, have begun mandating compliance by VoIP providers with emergency service location processes (911).
Within an enterprise network, there are two types of location information data that can be used to identify an emergency service location of IP telephony clients. The first type is an individual telephone number, such as in the form of a fixed direct inward dial (DID) number, that can be called externally and can be used to identify the individual's location down to the station or desk level. In this instance, each DID number and its fixed location information is stored in the corresponding ALI database. This arrangement works well for conventional Plain Old Telephone System (POTS) wired handsets because these telephones are not mobile and the ALI database can be updated to reflect the relocation or reassignment of DID numbers. The second type is an Emergency Location Identification Number (ELIN), which is a telephone number that can be dialed externally and can be used within a given area within the enterprise to identify a general location. An ELIN is typically implemented as a DID number. The ELIN, when called, will either ring a fixed telephone in the area described by the corresponding ERL or in some cases may be forwarded to the most recent emergency caller that used the ELIN. As such, an ELIN identifies an area's location at a larger level, such as at the floor level.
Since an individual telephone number provides more geographic resolution than an ELIN for locating a caller, it would be ideal to a have separate telephone number assigned to correspond to each IP telephone and have the telephone number and its fixed location information stored in a corresponding ALI database. Thus, for each IP telephone, the most precise location information would be available (i.e., the fixed location information). DID numbers, however, are expensive and typically a company or enterprise will purchase fewer DID numbers than the number of IP telephones that will be utilized. When an IP telephone is used with an assigned DID number, a corresponding entry in the ALI database indicating its expected or normal location is created, typically where the IP telephone was deployed initially. The mobility of IP telephones can quickly result in inaccurate data in the ALI database because all a user has to do is unplug the IP telephone and plug it in at another network jack or disconnect from one Wireless Access Point (WAP) and re-connect to another. The IP telephone will work, but the concordance between the location corresponding to the DID number and the new IP phone location is corrupt. In this scenario, therefore, the problem arises of how to locate an IP telephone when an emergency call is made from a location that is not the static location information stored in the corresponding ALI database. The DID number for that IP telephone should not be used, since the IP telephone is not located at the static location that is stored for that DID number. Further, if the general ELIN is used for all IP telephones on the enterprise network, the precision of the stored static locations corresponding to the DID numbers will be lost.
It is therefore desirable to have a method for providing to an emergency services location process the most precise location information available for an IP telephony client in a network where IP telephony clients may not be located at the static location stored in the corresponding ALI database.